Microorganisms belonging to the genus Bifidobacterium are one of the most dominant bacteria in the intestinal bacterial flora. The microorganisms themselves can be ingested, as in the case of lactic acid bacteria. It has been reported that ingestion of the microorganisms provides properties, for example, an action of regulating the functions of the intestines by conditioning a balance in the intestinal bacterial flora, an effect of improving the serum cholesterol level, and an immunostimulating action. There are reported agents for enhancing immune functions, among the above-described properties. Examples of the agent include an antitumor agent containing lactic acid bacteria or microorganisms belonging to the genus Bifidobacterium as an active component (Japanese Laid-Open Patent Publications No. 7-82158), an accelerator for production of cytokine (Japanese Laid-Open Patent Publications No. 8-92112), an agent for preventing and treating inflammatory bowel diseases containing microorganisms belonging to the genus Lactobacillus as an active component (Japanese Laid-Open Patent Publications No. 2003-73286), a granulocyte inducer containing cell walls and/or disrupted cell walls of microorganisms belonging to the genus Bifidobacterium as an active component (Japanese Laid-Open Patent Publications No. 7-330806), and a water-soluble immunostimulating substance containing microorganisms belonging to the genus Bifidobacterium as an active component (Japanese Laid-Open Patent Publications No. 9-241179). However, active components of all of these agents are bacteria themselves or polysaccharide fractions obtained by treatment with cell wall-digesting enzymes or ultrasonic disruption of bacteria.
On the other hand, regarding the immunostimulating action provided by an extracellular polysaccharide, it is reported as the follows: an antitumor agent containing a polysaccharide produced by Streptococcus lactis or Streptococcus cremoris as an active component (Japanese Patent No. 2678673), an antitumor activity of viscous capsular polysaccharides of Lactococcus lactis subsp. cremoris or Lactococcus lactis subsp. lactis (Japanese Laid-Open Patent Publications No. 1-277484), an anti-inflammatory agent or a bone marrow cell growth accelerator containing a polysaccharide obtained from a culture suspension of microorganisms belonging to the genus Lactobacillus as an active component (Japanese Laid-Open Patent Publications No. 7-70209), and a preventive effect of phosphorus-containing polysaccharides produced by Lactobacillus delbrueckii subsp. bulgaricus on autoimmune diseases (Japanese Patent No. 3017493). However, all of these reports relate to polysaccharides produced by lactic acid bacteria that have been separated from yoghurt or fermented milk.
Originally, the number of microorganisms belonging to the genus Bifidobacterium in the human intestinal bacterial flora is larger than that of lactic acid bacteria, and it seems that polysaccharides produced by the microorganisms belonging to the genus Bifidobacterium in the intestine have some biological defense functions. However, reported examples of polysaccharides produced by the microorganisms belonging to the genus Bifidobacterium are limited to polysaccharides containing only glucose produced by Bifidobacterium longum (Japanese Laid-Open Patent Publications No. 7-255465) and polysaccharides containing glucose, galactose, uronic acid, and other sugars produced by Bifidobacterium longum (Appl. Microbiol. Biotechnol. vol. 43, pp. 995 to 1000). Furthermore, their functions are not clear.